A typical cellular mobile radio telephone system consists of at least one mobile switching center (also known as a mobile telephone switching office), at least one base station, and at least one mobile station. The mobile switching center constitutes an interface between the radio system and the public switching telephone network. The base station transmits information between the mobile stations and the mobile switching center. Calls to and from mobile subscribers are switched by the mobile switching center. The mobile switching center also provides all signalling functions needed to establish the calls.
In order to obtain radio coverage of a geographical area, a number of base stations are normally required. This number may range from, in the exceptional case, one base station, and up to one hundred or more base stations in normal systems. The area is divided into cells, where each cell may either be serviced by a base station or may share a base station with a number of the other cells. Each cell has an associated control channel over which control (non-voice) information is communicated between the mobile units in that cell and the base station. Generally speaking, the control channel includes a dedicated channel at a known frequency over which certain information is communicated from the base station to mobile stations, a paging channel for unidirectional transmissions of information from the base station to the mobile stations, and an access channel for bidirectional communications between the mobile stations and the base station. These various channels may share the same frequency, or they may operate at different respective frequencies.
Each mobile station is assigned to one mobile switching center or home location register. The home location register is a database which contains information about all its assigned subscribers and where they are in the network. The home location register can be a stand-alone intelligent processor connected to one or more mobile switching centers or it can be part of a mobile switching center, possibly connected to one or more other mobile switching centers. When a mobile station enters a second mobile switching center service area to which it is not assigned, the new exchange is regarded as a visited exchange, and the subscriber as a visiting subscriber. Calls are now routed to and switched in this second mobile switching center.
Three types of transmissions normally take place on the control channels between the mobile stations and the base station, although other types are possible, such as an audit request and response, or order confirmation. First, when a mobile station is originating a call, it sends an access request to the base station the control channel of which has the strongest or second strongest signal. This access request serves to inform the base station that the requesting mobile needs to be assigned a voice channel over which the call can be connected. Second, when a mobile station is paged by a base station, indicating that the base station has a call to be completed to the mobile subscriber, the paged mobile station sends back a paging response on the access channel. Finally, when a mobile station travels from one cell to another, or for other reasons, the mobile station may send a registration access to identify itself and its presence to the telephone exchange associated with the cell.
An originating call access or a paging response is performed as follows. The mobile station scans the control channels of surrounding base stations and selects the one with the strongest or second strongest signal over which to make the access. The mobile station then performs the access by sending a transmission on the reverse control channel to the associated base station. The associated base station then passes the access or paging response to its mobile switching center.
A registration access is performed in cellular systems as follows. A registration access is an access requested by a mobile station to identify itself to a base station as being active in the system at the time the message is sent to the base station. The registration access may be requested for a number of reasons, for example: the mobile is switched on; the mobile determines that the time elapsed since the last registration has passed a specified limit; or the mobile detects a different system or area identification (SID or AID). The mobile then scans the control channels of surrounding base stations, and selects the one with the strongest or second strongest signal on which to complete the registration access, as explained above regarding call origination. The associated base station then passes the registration access to its mobile switching center.
For simplicity in the following discussion, an access request, paging response, and registration access of the type described above may be referred to as a mobile generated transmission when the discussion pertains to all three types.
Due to unfavorable (low) attenuation between remote base stations and mobile stations, it is possible that two or more base stations will receive a mobile generated transmission while only one base station is actually the intended recipient. In other words, it is possible that a mobile generated transmission which is intended for a given base station may be overheard by another base station operating on the same, or an adjacent, frequency. The risk of this increases as the number of cells, or base stations, in a given region increases to handle the increasing number of mobile stations.
Normally, protection codes are used which prevent the second base station from inadvertently overhearing the mobile generated transmission. However, there is only a small number of unique codes. Therefore, there is a good chance that various base stations may overhear mobile generated transmissions which are intended for other base stations. Conventional base stations are not capable of reliably determining whether such transmissions are actually intended for themselves or for other base stations.
In such cases, each of the base stations will try to act on the transmission received from the mobile station and will therefore notify the mobile switching center of a mobile generated transmission. Because two or more such transmissions are thus sent to the mobile switching center, the mobile switching center cannot accurately determine which of the mobile generated transmissions to accept and further process. Processing the mobile generated transmission received first in time is not necessarily correct because the transmission from the base station which was not intended to receive the mobile generated transmission may reach the mobile switching center first.
In one known system described in U.S. Pat. No. 4,481,670 to Freeburg, where more than one channel communication modules CCM receives an access request from a mobile, a general communications controller GCC is called upon to determine which CCM to use as the primary station. In this system described in this patent, the area is divided into seven zones, where each zone is covered by one or more transmitter/receiver pair. Each time a mobile transmits, signal strength readings are taken by each receiver hearing the transmission. These readings are used to compute an adjusted signal strength for each zone by multiplying the measured signal strengths for each zone by preselected factors associated with the particular zone. The GCC then selects the zone which has the largest adjusted signal strength for a particular transmission from the mobile as being the zone in which the mobile is most likely located. The selected zone is then stored for later reference when it becomes necessary to locate the mobile.
Whenever a message signal together with an average signal strength measurement is received by the GCC from a CCM, a message timer is set to provide a time interval during which the same message signal is received by other CCM's and sent together with an average signal strength measurement to the GCC. All signals from other CCM's received within the set time period are used to determine where the mobile is located. Thus, the location of each mobile is updated each time a message signal is received by the CCMs. Then, when it is desired to transmit a message signal for the GCC to a selected mobile, the most recently determined location is used as a first try for successful connection.
According to this patented method, the system described in the patent appears to be one in which adjacent CCM's operate on the same frequencies, thus permitting adjacent CCM's to hear the same message from a mobile. Further, only the largest adjusted signal strength is used, with no provision for the possibility that two signal strengths may be so close as to preclude an accurate determination. Finally, by multiplying the signal strength measurements by predetermined factors, inaccurate determinations of locations are possible.
In some conventional systems, the mobile switching center automatically starts processing of the first mobile generated transmission. However, in certain circumstances, this may not be appropriate and may result in lost calls. For example, consider the case when a mobile station makes an access in a network where adjacent base stations operate on different frequencies. Because the receivers in the control channels are not normally receiving any informative signals, distant control channel picks up a weak whisper of this access from far away. The base station with which this distant control channel is associated then sends this weak access to the mobile switching center. The weak access, because it occurs first in time is processed. Thereafter, if the intended base station forwards the access request it has received, the call is lost because the mobile switching center has reacted to the first received request and therefore ignores the second.